Changing the tuna - warming seas and the expanse of the bluefin

A study revealing the unusual presence of bluefin tuna in warming Greenlandic waters is to be presented at this year’s Annual Science Conference in A Coruña.

Published:
4 September 2014

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​The paper, published in the August 2014 edition of the Global Change Biology Journal, will constitute a scientific talk as part of the ASC's Theme Session J, ‘Climate Change: Back to the future for marine predators’. It hypothesizes that warming ocean temperatures and the ensuing alteration in the spatial distribution of its prey could be responsible for the summer migration of bluefin tuna to the Atlantic Ocean east of Greenland – a high-latitude subpolar area previously thought to be outside the marine predator’s common range.

A curious catch

When local fishermen pulled their net out of the Denmark Strait one day in August 2012 whilst carrying out an exploratory fishery for Atlantic mackerel they got more than they bargained for. Three large bluefin tuna, each weighing around 100 kilogrammes and therefore likely adults, were emptied onto the boat’s deck amongst six tonnes of the smaller mackerel upon which they prey. This was an anomaly: up until this point there had been no scientifically confirmed sightings of the species, with only a 1671 explorer’s tale, a 1900 stranding, and an instance of a detached electronic tag coming close.

This prompted DTU Aqua professor Brian MacKenzie along with senior scientists Mark Payne and Jesper Boje, Jacob Højer from the Danish Meteorological Institute, and Department Head Helle Siegstad from the Greenland Nature Institute (GNI) to look into what might have led to the appearance of the important food fish in such northerly waters. With shared interest in the spatial distributions and migratory patterns of various fish species and how these are impacted by climate change the scientists pooled their knowledge to investigate. What, they contemplated, could have driven bluefin tuna to these new grounds?

A matter of degrees

Typically, the sleek, streamlined apex predators feed and school in waters at least 10-11°C, and most commonly several degrees higher. Nuances in temperature MacKenzie et al argue, represent the driving force behind the extended natural range of the tuna. And it was the temperature of the water that day in August 2012 that held the key.

Using Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) satellite imagery and in situ instruments, the sea surface temperature of the waters in which the tuna were caught, the Denmark Strait-Irminger Sea area, was shown to have been rising steadily. The degree reading was higher in August 2012 than at any time since 1870, and between 1985-1994 and 2007-2012 the expanse of water with temperature greater than or equal to 11°C grew by 720 000 km², an area larger than Texas. These conditions combined to make the region hospitable for the bluefin tuna.

Prey moving north

Although more physiologically tolerable temperatures have laid the foundation for the tuna being able to exist at higher latitude than before, other factors, including prey abundance, have played a significant part. One of the bluefin tuna’s prey species, the mackerel that accompanied it in the fishermen’s nets, has similarly shifted its range from the northwest European continental shelf towards the Faroe Islands and south Icelandic shelf.

Another dynamic which may have catalyzed the occurrence of the tuna near Greenland is the overall numbers of the species itself, with the eastern Atlantic and Mediterranean Sea biomass increasing over the last 3-5 years as a knock-on effect of fishery management regulations. The ultra-mobile tuna have high dispersal potential and therefore it’s possible that the fish have spread out to reduce competition for prey in more densely-populated areas.

The conclusion reached was that higher sea temperatures have been progressively restructuring the foodweb in east Greenland waters through improved thermal conditions for migratory prey.

Climate change in focus at ASC

The study, which details in depth the data and discusses key questions relating to the migratory origins of the tuna caught in Greenland, will form part of an important theme session topic at the Annual Science Conference which kicks off on 15 September in the Spanish coastal city of A Coruña.

‘Climate change: back to the future for marine predators’, Theme Session J, will take place from 09.00-17.00 on Friday 19 September in the Palexco Congress and Exhibition Centre and will feature, alongside the tuna study, presentations on food resources in the Barents Sea, climate influence on marine birds in the northeast Atlantic, and the impact of salt water inflows to the Baltic Sea on cod reproduction.